According to experts, the temperature has plummeted and clouds of fog and smoke hang in the sky blocking sunlight for several days in Bangladesh, India and Nepal.
Normal lives have been affected with many flights diverted and suspended and trains delayed because of low visibility.
Experts have also said they have noticed that the intensity of smog has grown in the Indo-Gangetic plains in the last few years, leading to increased impacts.
"Since 1990 onwards, there has been increase in the number of smog-affected days in northern India," the BBC quoted BP Yadav, director of the Indian Meteorological Department as saying.
"It is not a linear trend showing an increase every year. There are, of course, year-to-year fluctuations.
"But there are more years that have seen dense fogs," he said.
India, like Bangladesh, sees lots of constructions during winter as this is the dry season before the region gets monsoon rainfall preventing such works.
"Construction works too are major contributors for the smog in this season as they lead to more pollution in the air," Yadav said.
In its World Energy Outlook 2010, the International Energy Agency said that "India is the second-largest contributor to the increase in global energy demand to 2035, accounting for 18 percent of the rise."
Scientists say pollutants and aerosols in the air enhance condensation of water in the atmosphere causing dense smog.
"The more pollutants in the air, the denser the smog," Keshav Prasad Sharma from Nepal's Department of Hydrology and Meteorology, said.
"In some Nepal-India bordering areas, smog blankets can be seen from early evening," he said.
When such blankets of smog block sunlight, sending temperatures down, people make fire from wood, cow-dung cake and hay to warm themselves and that creates more air pollution that leads to denser smog.
Scientists say the real trouble is that smog during winter cannot escape to the upper atmosphere as it can during other seasons, because of meteorological conditions.
"During winter, the cold air that blows towards the southwest from the northeast tends to push the boundary layer the layer of atmosphere closest to the Earth surface low," William Lau, deputy director for atmospheres at NASA's Goddard Space Flight Center, said.
"As a result, all the pollutants get trapped in the boundary layer that is pushed down to as low as one kilometre from the Earth's surface while it is more than five kilometres away during other seasons.
"The cold wave becomes severe because of this local trapping of the aerosols and other pollution that block off the solar radiation and create very unhealthy air in this part of the world," he added.